Improve and consolidate documentation of various writeSequence() implementations

Description

As discussed in #6083, some of the writeSequence() implementations aren’t very clear about the datatype it accepts. For example twisted.internet.abstract.FileDescriptor vs. twisted.internet.interfaces.ITransport.writeSequence.

Quoth Glyph:

"""
If it is super, crystal clear that you absolutely must always pass a L{list} of L{bytes}es, then it will be a bit less likely that people will bump into that problem, and the documentation should be fixed regardless of whatever other error-behavior improvements are done.
"""